New issue 927: Field parameter registry https://bitbucket.org/yt_analysis/yt/issue/927/field-parameter-registry
I think we should have some sort of field parameter registry. Now that field parameters can have units, we need to somehow know what the units are when we mock up the field parameter during field detection.
I think the easiest way to do this would be to explicitly add user-defined field parameters to a `dataset` instance after it is loaded in memory, sort of like particle filters.
Before creating a full YTEP for this, I'm opening this issue to facilitate discussion and to check if people think this is a good idea.